It has been reported that the low-molecular-weight organic electroluminescent (OLED for short hereinafter) device is provided with a copper phthalocyanine (CuPC) layer as the hole injection layer so as to improve the initial characteristics, such as reduced drive voltage and increased light-emitting efficiency, and to improve the life characteristics (Non-Patent Document 1: Applied Physics Letters, U.S., 1996, vol. 69, p. 2160-2162).
It has also been reported that the organic electroluminescence (PLED for short hereinafter) device with a polymeric luminescent material produces the same effect as the OLED device does if it is provided with the hole transport layer in the form of thin film of polyaniline-based material (Patent Document 1: JP-A 3-273087; Non-Patent Document 2: Nature, U.K., 1992, vol. 357, p. 477-479) or polythiophene-based material (Non-Patent Document 3: Applied Physics Letters, U.S., 1998, vol. 72, p. 2660-2662).
It has recently been found that a charge transport varnish in the form of homogeneous solution completely dissolved in an organic solvent can be produced from a highly soluble low-molecular-weight oligoaniline-based material or oligothiophene-based material. Reportedly, this varnish is made into the hole injection layer to be placed in the organic electroluminescence (organic EL for short hereinafter) device so that the underlying substrate becomes flat or the resulting EL device exhibits good characteristic properties (Patent Document 2: JP-A 2002-151272; Patent Document 3: WO 2005/043962 pamphlet).
The low-molecular-weight oligomer compound intrinsically has such a low viscosity that it gives rise to a solution of ordinary organic solvent which presents difficulties in forming a highly uniform film by coating (such as spin coating, ink jet coating, and spray coating) and baking under various conditions owing to its narrow process margin.
Nevertheless, it has become possible to form a highly uniform film by various coating methods as the result of using additional solvents for adjustment of viscosity, boiling point, and vapor pressure (Patent Document 4: WO 2004/043117 pamphlet; Patent Document 5: WO 2005/107335 pamphlet).
As mentioned above, it has recently become common practice to use a low-molecular-weight oligomer compound for the hole injection layer in the organic EL device.
However, the low-molecular-weight oligomer compound is still required to improve in solubility so that it easily adapts itself to various coating methods, such as spin coating, ink jet coating, and spray coating. It is also required to improve further in conductivity as well as in light emitting efficiency and luminance characteristics which are necessary for organic EL devices.
Particularly, improvement in light emitting efficiency and luminance characteristics is important to both OLED devices and PLED devices.
The characteristic properties required of the hole injection layer include ability to inject holes into the luminescent layer, ability to block electrons from the luminescent layer, and ability to prevent deactivation of excitons in the luminescent layer. These functions greatly affect the light emitting efficiency and luminance characteristics of the organic EL device mentioned above. Consequently, there is a demand for a low-molecular-weight oligomer compound for the hole injection layer that realizes outstanding functions.
Incidentally, it has been reported that the organic EL device has an extended life if it has a thin film formed from a composition of conductive polymer (such as polystyrenesulfonic acid and polyaniline) incorporated with a silane compound (see Patent Document 6). However, nothing has been reported about an instance in which a silane compound is added to the charge transport varnish containing a low-molecular-weight compound.                Patent Document 1: JP-A 3-273087        Patent Document 2: JP-A 2002-151272        Patent Document 3: WO 2005/043962 pamphlet        Patent Document 4: WO 2004/043117 pamphlet        Patent Document 5: WO 2005/107335 pamphlet        Patent Document 6: JP-A 2003-45667        Non-Patent Document 1: Applied Physics Letters, U.S., 1996, vol. 69, p. 2160-2162        Non-Patent Document 2: Nature, U.K., 1992, vol. 357, p. 477-479        Non-Patent Document 3: Applied Physics Letters, U.S., 1998, vol. 72, p. 2660-2662        